I can't understand why all of the common NVDs mount in front of the standard "daytime" rifle scopes: Doesn't this require extra optics to focus the image on the intensifier and then back it out to the original focal distance so the day scope can make use of it? Why wouldn't it be simpler and cheaper to put the NVD on the ocular side? That way it doesn't constrain the day scope's objective size and it can take advantage of the inherent light-gathering of the day scope? (I understand this would require mounting the day scope forward, but either the NVD can pass-through daylight or else optics have to be moved around anyway to make use of the NVD.)
Night Vision Why do NVDs mount in front of scopes?
- Thread starter dbooksta
- Start date
Eye relief possibly?
The short answer is this, mounting a NVD in front of a day optic leaves said day optic in place and zeroed, in addition your cheek weld remains constant. By putting the NVD IN the back of the scope with a removable cartridge optic for day/night capability (and there are such devices) you loose a ton of light going through the day optic. I know it seems like the scope would gather more light but it really doesn't. Hanging a mono like a PVS 14 off the back can be done but you loose even more light gathering capabilities. This can be done but it is not optimal. Plus you move your cheek weld back because of the length of the device.
Clip ons, NVDs that go in front of day optics are purpose designed for the task of turning a day optic into a night optic. There is a HELL of alot going on inside them in terms of lens elements and prisms and collimation and... Clip ons aren't perfect but they are a lot better than anything else.
A few reasons.
When the clip on is in front of your day scope, you are allowing it to receive and amplify more existing light. That is why clip ons have catadioptic lenses inside of them (just like a telescope). That in turn, gives you a brighter more crisp image, more FOV, etc. I'm not debating that daytime scopes aren't designed to bring in light, however, they are not whats allowing you to see in the dark. Also, the way clip on's are designed, they work with a large majority of existing daytime scopes. The focusing isn't that technical. There is a large focus knob on the clip on, that you twist until your image crisps up. Same as your daytime optic. It's one more step, but it would be a similar process if the nv were behind the daytime optic.
If you a put night vision device behind a daytime magnified optic, there are a few issues. There isn't really any universal way to mount them, meaning it's not just plug and play-so to speak- like a clip on is. There is also an amount of light degradation, because every time light passes through glass it is degraded slightly. The tube, the key ingredient to the mix, that's doing all the light amplifying is now hindered from amplifying as much existing light as it should be able to because its the very last to get the light. Dont get me wrong, you can still mount a PVS-14 behind a RDO or a ACOG (non tritium) or even a 1-4 or 1-6 variable, but it isn't optimum with those higher magnified optics, and wont get you out to the same distances in a practical manner. Eye relief is another issue. It's hard to get it just right on the rail, and when you do, you find that your daytime optic is pushed far forward on the rail, and now you have a wonky looking setup, and you really aren't going to be able to use in the daytime, because you now have a daytime eye relief issue.
When the clip on is in front of your day scope, you are allowing it to receive and amplify more existing light. That is why clip ons have catadioptic lenses inside of them (just like a telescope). That in turn, gives you a brighter more crisp image, more FOV, etc. I'm not debating that daytime scopes aren't designed to bring in light, however, they are not whats allowing you to see in the dark. Also, the way clip on's are designed, they work with a large majority of existing daytime scopes. The focusing isn't that technical. There is a large focus knob on the clip on, that you twist until your image crisps up. Same as your daytime optic. It's one more step, but it would be a similar process if the nv were behind the daytime optic.
If you a put night vision device behind a daytime magnified optic, there are a few issues. There isn't really any universal way to mount them, meaning it's not just plug and play-so to speak- like a clip on is. There is also an amount of light degradation, because every time light passes through glass it is degraded slightly. The tube, the key ingredient to the mix, that's doing all the light amplifying is now hindered from amplifying as much existing light as it should be able to because its the very last to get the light. Dont get me wrong, you can still mount a PVS-14 behind a RDO or a ACOG (non tritium) or even a 1-4 or 1-6 variable, but it isn't optimum with those higher magnified optics, and wont get you out to the same distances in a practical manner. Eye relief is another issue. It's hard to get it just right on the rail, and when you do, you find that your daytime optic is pushed far forward on the rail, and now you have a wonky looking setup, and you really aren't going to be able to use in the daytime, because you now have a daytime eye relief issue.
I've mounted my Envis behind my XTR with a GG&G QD mount for a PVS14. The scope is mounted slightly forward, but that gives me nose-to-charging handle eye relief with NV off. Slap it on real quick, move stock back a notch and is gtg. It may not be what some consider "optimal" but it's fine for what I want it for.
Question for you, am I right to assume that there are significant differences between image intensifier clip-ons and thermal clip-ons (besides the obvious difference in technology basis). Being that thermal is a processed image that can be shown on a screen pretty much wherever without regard to the axis of the objective lens and detector, wouldn't that GREATLY simplify the process of boresighting a thermal device vs. an I^2 device? The manufacturer (or the end user in the case of the IR Hunter), can simply adjust the image placement on the screen until it is "boresighted?" Based on your post above and the fact that image tubes (as far as I know) are straight pass-through devices which would require optical manipulation to boresight, would I be correct to assume that there is much more that can go wrong, boresight wise, with an I^2 clip-on than with a thermal clip-on? I own a thermal clip-on but have never owned or used a NV clip-on, this is pure curiosity.
Cuz day opitc don't have enough diameter to provide enough light source to the rear mounted NVD. Exp. 
VS-14's 3x magnifier have 53mm diameter lens to provide enough light source, but a day optic with 50mm diameter lens always have 10+ magnificent, that means if you put a NVD behind those types of day sights, you only get less than 30% performance of your NVD, got it bro?
VS-14's 3x magnifier have 53mm diameter lens to provide enough light source, but a day optic with 50mm diameter lens always have 10+ magnificent, that means if you put a NVD behind those types of day sights, you only get less than 30% performance of your NVD, got it bro?
